This application is based on patent application no. 964,817 filed in Finland on Dec. 2, 1996. The content of the Finnish priority application is incorporated herein by reference.
The invention relates to a method of maintaining a group call in a mobile communication system comprising:
a network infrastructure, and mobile stations that communicate with the network infrastructure over a radio path, whose frame structure comprises time slots,
the method comprising the steps of:
establishing a group call in which a first mobile station is prioritized, the mobile stations that participate in the group call communicating in speech items, and the frame structure comprising time slots that a second mobile station sending a speech item employs in its speech item.
The method of the invention is intended for use particularly in PMR (Private Mobile Radio or Professional Mobile Radio) networks, or trunking networks, which are typically company networks or private mobile radio networks used by authorities, all the channels in these networks being allocated to one or more companies or authority organizations. In addition to subscriber numbers, the subscribers in these networks have been assigned group numbers indicating to which group call group or subscriber group the subscriber belongs. It is thus possible to switch calls directed to the members of a certain group to all the subscribers of this group.
A group call is one of the key functions in a PMR network. A group call is used when several participants are involved, particularly when an entire group must continuously know how things proceed. A group call is a conference call in which all participants can take turns to speak, or transmit speech items, and to listen to each other. In group calls the whole group is called by one group identity code. It is known that many mobile telephone systems, particularly systems used by companies and authorities, apply group calls. As regards the radio path, a group call is typically implemented in simplex form as a point-to-multipoint call, in which speech is transmitted from one talking party to multiple talking parties, and a speech item is allocated to a next talking party according to a predetermined practice. It is naturally also possible to implement a group call as a duplex call.
A call or a group call is here taken to mean complete exchange of information between two or more parties. A call may consist of one or more speech items. In a semi-duplex call, these speech items are sequential ones.
The invention is applicable in mobile communication systems with either digital or analogue radio paths. Analogue mobile communication systems are described, for example, in MPT 1327, A Signalling Standard for Trunked Private Land Mobile Radio Systems, January 1988, revised and reprinted November 1991, and MPT 1343, Performance Specification, January 1988, revised and reprinted September 1991, both issued by the Radio-communications Agency, published by the British Department of Trade and Industry.
An example of a digital mobile telephone or mobile communication system in which the invention can be applied is the TETRA system (Trans-European Trunked Radio). Its implementation is described in the standard ETS 300 392-2, Radio Equipment and Systems (RES); Trans-European Trunked Radio (TETRA); Voice plus Data (V+D) Part 2:Air interface (AI), Mar. 20, 1996, ETSI, 583 pages. In this system, the allocation of speech items is controlled by a Switching and Management Infrastructure (SWMI) through base stations connected to it.
In FDMA systems (Frequency Division Multiple Access), the facility of sending signalling messages or other data during calls or data calls is called in-band signalling. In this system, part of the transmission capacity of the channel is typically used during the call for signalling that supplants actual speech or circuit-switched data.
WO A1 91/09481 describes traffic between a first and a second communication unit in a quasi-duplex situation, for example on a direct mode channel. The first communication unit transmits a signal with periodically-occurring holes therein, the signal also comprising a speech information portion and information packets that inform of the location of the next hole. The receiving, second communication unit sends an interrupt request signal to the first communication unit in the hole indicated by the information packet. The first communication unit receives the interrupt request signal and processes it, and if the first communication unit determines that the interrupt request signal complies with (desired) predetermined criteria, it sends an acknowledgement to the receiving second communication unit. The acknowledgement informs the second communication unit that the first communication unit will interrupt transmission (for a predetermined period of time) and thereby allow the receiving second communication unit to start a transmission. The second communication unit can thus start the transmission.
U.S. Pat. No. 5,025,442 teaches a quasi-duplex mobile telephone system employing a control time slot during which a receiving communication unit is allowed to interrupt the transmission of a transmitting communication unit by sending the transmitting communication unit an interrupt signal by which the transmitting communication unit is instructed not to transmit.
It is also known that part of the traffic capacity allocated to a user is permanently allocated for signalling. This procedure is called out-band (channel-specific) signalling.
In common mobile telephone networks, such as NMT (Nordic Mobile Telephone) or GSM (Global System for Mobiles) systems, typically all calls have the same priority, i.e. no user is prioritized over any other user.
In private radio networks, i.e. trunking networks, the situation is different. The operation of these PMR networks can be arranged in such a way that users performing certain typexe2x80x94e.g. emergencyxe2x80x94servicing or other important duties have a higher priority, i.e. their high priority call is switched first in the event of congestion.
An example for a high priority call is an emergency call, i.e. a call established when at least one subscriber needs immediate help. The subscriber in an emergency can activate an emergency call, or the activation can be performed by some other subscriber station or by a dispatcher controlling the operation. The emergency call can be addressed to a talk group, i.e. a group call, whereby the members of the talk group need not just listen to the conversation but they can also participate in it. In other words, the members send speech items. This is often necessary, for example, when instructions are given to a group member in an emergency. However, the subscriber requiring a communication connection in an emergency should always be allocated a speech item as soon as possible in a group call where several users take turns to speak.
In a mobile telephone system based on a trunking method, the mobile station, however, has to request a speech item from the system, and the system allocates the speech item in order of time or priority. The system is also able to interrupt an ongoing speech item.
A problem in the prior art is that the control channel capacity of a prior art mobile communication system protocol, particularly of the TETRA mobile telephone system, is very limited during speech items in individual calls of a single base station and in group calls at the base station that the subscriber station sending a speech item is attached to. During the speech item, only the 18th frame of the TETRA frame structure has available a Slow Associated Control Channel, i.e. a once-a-second time slot. Particularly in private mobile radio networks which are used by authorities and in which emergency group calls are sent frequently, it is essential that a subscriber or mobile station in an emergency is given a chance to request a speech item sufficiently often during a speech item sent by a subscriber participating in a group call, so that the speech item request is reasonably likely to be successful.
In prior art solutions, particularly in the TETRA system, a Slow Associated Signalling Channel is used so that all mobile stations participating in a group call can request a speech item using a random access method, such as a first-try procedure, by sending speech item request signals.
A problem arises when a second mobile station is sending a speech item and several third mobile stations participating in an emergency group call, as well as a first mobile station actually in an emergency, simultaneously request a speech item in the time slot of the 18th frame reserved for a speech item request, or in a part of the time slot. It is then likely that it is one of the third mobile stations that succeeds in sending a speech item request, rather than the first mobile station, i.e. the one in an emergency. For example, a transmission request from a third mobile station may collide with the speech item request sent by the first mobile station. The first mobile station, i.e. the one in an emergency, is then not able to deliver its speech item request to the network infrastructure, and so a speech item will not be allocated to the first mobile station in an emergency.
One way of solving the problem is to allocate a separate uplink channel, time slot or frequency to a prioritized mobile station, so that a speech item request can be delivered immediately. A problem of this solution is that it wastes radio resources. It should be noted that when a subscriber is in an emergency or is prioritized, the traffic density is also otherwise likely to be higher than normal. In addition, allocation of a separate uplink channel to a prioritized user is a complicated non-standard procedure for a network infrastructure.
Another way of solving the above problem is to allow a prioritized mobile station to send a speech item request on a Common Control Channel. A problem is that a prioritized mobile station has to wait for its transmission turn on the control channel, and that in so doing the prioritized mobile station is not on a traffic channel, whereby it may lose a call or at least not receive part of the transmission of a transmitting mobile station.
Another problem in the prior art solutions is that when a speech item is terminated in a prioritized group call, there begins a random access time, during which the mobile stations participating in the group call can request a speech item. It may then happen that the speech item requests of the third mobile stations collides with the speech item request made by the first, prioritized mobile station, and the first mobile station cannot therefore be quickly allocated the speech item it needs. This has particular significance in private mobile radio networks used by authorities, where it must be ensured that a speech item request from a prioritized mobile station, potentially in an emergency, is reasonably likely to succeed.
The object of the present invention is to solve the above problem of the prior art.
The object of the present invention is to provide a method by which it can be ensured that speech item requests sent by a prioritized mobile station, e.g. a mobile station in an emergency, participating in a prioritized call actually reach the network infrastructure, which can allocate a speech item precisely to the prioritized mobile station after receiving the speech item request.
This new method of maintaining a group call is achieved with a method of the invention, the method being characterized by further comprising the steps of:
allocating only to the first mobile station time slots or time periods in which only the first mobile station may send signalling messages to said network infrastructure,
sending the first mobile station and the third mobile stations data indicating in which time slots or time periods only the first mobile station may send signalling messages to said network infrastructure,
the third mobile stations refraining from sending their signalling messages in said time slots or time periods allocated to the first mobile station.
The invention also relates to a mobile communication system comprising:
a first, a second and third mobile stations, a mobile exchange for relaying group calls and transmissions and for maintaining a group call, said mobile stations that participate in said group call communicating in speech items,
a base station for maintaining a frame structure comprising time slots that the second mobile station sending a speech item employs in its speech item.
The mobile communication system of the invention is characterized by further comprising:
allocation means for allocating time slots or a time period only to the first mobile station so that only the first mobile station can send signalling messages in certain time slots or time periods to said mobile exchange,
transmission means for sending the first mobile station and the third mobile stations data in a control message indicating in which time slots or time periods only the first mobile station may transmit,
control means in said third mobile stations for preventing transmission of signalling messages in said time slots or time periods allocated to said first mobile station.
The invention also relates to a mobile station of a mobile communication system, communicating in a group call, the mobile station comprising: a transceiver for sending transmissions in a frame structure comprising: time slots that a second mobile station sending a speech item in the group call employs in its speech item, and time periods that are common to the mobile stations that are not sending a speech item, for transmitting signalling messages to the network infrastructure; a control unit; a user interface; and a memory unit.
A mobile station of the invention is characterized in that said mobile station comprises a control unit, responsive to a control message sent by said mobile communication system, for preventing transmission of signalling messages in the time slots or time periods allocated to a first mobile station and indicated in said control message.
A mobile station of the invention can also be characterized in that, in response to a control message sent by said mobile communication system, the transceiver of the mobile station is arranged to send a signalling message during the time period indicated in the control message.
The invention is based on the idea of ensuring that a participant of a prioritized group call, for example a mobile station that has activated an emergency call, is assigned signalling capacity at certain intervals, so that said mobile station has a chance of sending a speech item request, and on the other hand so that the mobile station sends the system an indication of its presence on a continuous basis. This is done by allocating a random access time slot of the frame structure to the first, prioritized mobile station, and by preventing the other, third mobile stations that participate in the same call from transmitting during a time period of the frame structure that is allocated to the first mobile station. In the invention, the first, prioritized mobile station, which is the one in an emergency, and the third mobile stations, participating in the prioritized group call, are sent data indicating the time slots or time periods in which only the first mobile station is allowed to send signalling messages to said network infrastructure.
The advantage of the method, mobile communication system and mobile station of the invention is that the invention ensures that, for example, a mobile station in an emergency, particularly a prioritized mobile station, can send a speech item request, and that when the mobile station sends the speech item request, the request also reaches the network infrastructure, which is then able to allocate the next speech item to the prioritized mobile station, for example to one in an emergency.